1. Field
The present invention relates to a superconducting power system and a superconducting cable installation method. More particularly, the present invention relates to a superconducting power system which is capable of effectively absorbing an axial force caused by the contraction of a superconducting cable when the superconducting cable is cooled and in which the superconducting cable is installed in the form of minimizing unnecessary waste of an installation space, and a method of installing a superconducting cable.
2. Description of the Related Art
A superconducting wire has an electrical resistance approaching ‘0’ at a certain temperature and may thus have high current transfer capability even at a low voltage.
In the case of an alternating current (AC) superconducting cable which is a power cable having high-capacity, high efficiency, and eco-friendly power transmission capability, a ratio of a charging current to an allowable current is small. Thus, the AC superconducting cable is less restricted in terms of a distance of power transmission using the charging current than existing cables. Thus, the AC superconducting cable is advantageous in terms of long-distance power transmission. In particular, a direct current (DC) superconducting cable has an electrical resistance approximating ‘0’ and thus does not exhibit a voltage drop during long-distance power transmission.
For a superconducting cable including the superconducting wire, a cooling method performed using refrigerant such as nitrogen to form and maintain an extremely low temperature environment and/or an insulating method of forming a vacuum layer is employed. Since the superconducting cable is maintained at extremely low temperature using refrigerant, an intermediate joint box may be installed in an installation section at predetermined intervals and a terminal joint box may be provided to be connected to a power system in a normal-temperature environment.
In general, a superconducting cable of a superconducting power system includes an inner metallic conduit and an outer metallic conduit.
A cooling unit in which liquid refrigerant circulates may be provided at an inner side of the inner metallic conduit of the superconducting cable, and a multi-layer insulation (MLI) type insulating layer may be provided outside the inner metallic conduit to prevent the transfer of heat through radiation or the like.
For vacuum-insulation, a vacuum unit having a space with spacers may be provided at an outer side of the insulating layer. The outer metallic conduit may be located at an outer side of the vacuum unit.
The inner metallic conduit and the outer metallic conduit may be each formed of a metal material, e.g., an aluminum material.
In general, a wire of an ultra-high voltage cable expands by heat generated by a conductor during transfer of electrical current. Thus, in a cable duct, distortion may occur and a force may be concentrated onto a fixed portion thereof. Therefore, methods of installing a cable using a technique of cable-laying design have been developed under various environmental conditions. Ultra high voltage cables have been developed for a long time and thus techniques of cable-laying design have also been accumulated.
However, the superconducting cable may contract due to use of liquid nitrogen which is extremely low temperature refrigerant, unlike conventional ultra high voltage cables. In particular, since the inner metallic conduit accommodates liquid refrigerant, contraction may occur to a serious extent due to extremely low-temperature liquid refrigerant circulating in the cooling unit.
In contrast, the transfer of heat to the outer metallic conduit through radiation, convection, conduction, or the like is blocked by an insulating unit and a vacuum unit. Thus, a degree to which the outer metallic conduit contracts due to the liquid refrigerant is not high.
Thus, the inner metallic conduit and the outer metallic conduit, which are provided to maintain the hardness, etc. of the superconducting cable, contract to different extents when the liquid refrigerant is filled or circulates therein.
Each of the inner metallic conduit and the outer metallic conduit has a corrugated structure. Thus, when the inner metallic conduit contracts, the whole superconducting cable contracts. A contractile force of the contracting superconducting cable may be applied to each intermediate joint box or the terminal joint box.
When the contractile force is applied to each joint box due to the contraction of the superconducting cable, a problem or an accident related to connection to each joint box may occur. To solve this problem or accident, in the related art, each intermediate joint box or the terminal joint box may be configured to be slidable within a predetermined range in a direction parallel to a contractile force applied thereto rather than being configured to be fixed devices. Otherwise, a horizontal detour space or a horizontal offset space may be formed in a superconducting cable laying area to set a section through which the superconducting cable may detour to a large extent in a horizontal direction through this space, thereby preventing problems caused by the contraction of the superconducting cable.
However, the system stability may be lowered when each joint box is installed to be slidable. A superconducting cable laying area in a superconducting cable laying section may unnecessarily increase and an axial force may not decrease to a sufficient extent due an internal frictional force when the offset space is formed.
In particular, when the superconducting cable is installed in the earth, a sufficient offset space is not easy to secure and interference may occur with other cables installed together with the superconducting cable in a cable-laying space.